1. Field of the Invention
This present invention relates to a digital composite color video signal separating circuit with a vertical correlation weighting function, and more particularly, to a composite signal separating circuit in a color television receiver.
2. Description of the Related Art
In an existing color television system, a carrier chrominance signal C is superimposed on a luminance signal Y to provide a composite video signal. For this reason, color television receivers need a circuit adapted to separate the composite video signal into the luminance signal Y and the carrier chrominance signal C, that is a Y/C separating circuit.
Owing to recent advances in digital techniques, conventional Y/C separating circuits of analog type have been developed into digital Y/C separating circuits for separating the luminance and chrominance signals after conversion of the composite picture signal to digital form.
For example, conventional analog Y/C separating circuits use a low pass filter for separation in a horizontal direction of the composite video signal, with the result that the horizontal resolution of the luminance signal is reduced considerably. To eliminate such a drawback, Y/C separation circuits have been developed which perform, after conversion of the composite color signal to a digital signal, arithmetic operations of the digital signal with respect to the vertical direction of a television picture by the use of line memories.
FIG. 5 shows a conventional digital Y/C separating circuit.
An input circuit 401 is connected to receive a digital composite color signal provided from an analog-to-digital conversion circuit (not shown) which performs the analog-to-digital conversion of an analog composite color signal by means of a clock signal with a frequency of 4fsc (fsc=color subcarrier frequency), for example. The digital composite color signal is applied to a line memory 402, which delays the composite color signal by 910 samples, and an adder 405. The output of line memory 402 is coupled to inputs of adders 405, 406 and 404 and a line memory 403.
Adder 405 calculates a difference between the input digital composite color video signal and the delayed digital composite color video signal from line memory 402 and then feeds the result to adder 407. Adder 406 calculates a difference between the one-scanning-line delayed (910-sample-delayed) composite color signal from line memory 402 and the two-scanning-line delayed (2+910-sample-delayed) composite color signal from line memory 404 and provides the result to adder 407. An output signal of adder 407 has its unwanted components removed by a bandpass filter 408 and is then led to an output terminal 410 as a digital chrominance signal (C signal). This is because, in the NTSC system, subcarriers on the upper or lower lines against a line are opposite in phase to each other. The output of bandpass filter 408 is coupled to a minus input of adder 404 so that the luminance signal (Y signal) is separated from the composite signal and taken from an output terminal 409.
With the digital Y/C separating circuit, as described above, the Y signal and C signal can be separated with little degradation of the resolution of the luminance signal.
In the NTSC system which is one of existing color television systems, a limitation on the horizontal band-width of the chrominance signal is standardized, but no vertical bandwidth limitation exists. Therefore, the chrominance signal may mix with the luminance signal. In the digital Y/C separating circuit, the leak of the chrominance signal in the luminance signal may produce dot crawl which degrades the picture quality. When an oblique component in the luminance signal is high, the luminance signal leaks in the chrominance signal, resulting in cross-color.
The above problems will be discussed in detail with reference to FIG. 6. Assume now that composite-signal waveforms a, b and c on three scanning lines abruptly vary between a and b in the vertical direction. In the conventional digital Y/C separating circuit using line memories, subtractions are performed between the waveforms a and b and between the waveforms b and c, and the results are added together. As can be seen from a waveform d, therefore, the vertical abrupt variations in the composite-signal waveforms make the sufficient separation of the C signal and the Y signal difficult. As a result, the C signal will leak in the Y signal, resulting in the dot crawl. Further, the Y signal will also leak in the C signal, thereby producing the cross-color.
As described above, the conventional digital Y/C separating circuit using line memories can prevent the degradation in the resolution of the luminance signal. However, if the chrominance signal and the luminance signal have a wide bandwidth, then the picture quality will be degraded because of the cross-color and the dot crawl due to the imperfect separation of the signals.